Weener said the throttle went to idle six seconds before the derailed train came to a complete stop — "very late in the game" for a train going that fast" — and the brakes were fully engaged five seconds before the train stopped.

He said investigators were also examining the engineer's cellphone — apparently to determine whether he was operating the train while distracted.

Asked whether the tragedy was the result of human error or faulty brakes, Weener said: "The answer is, at this point in time, we can't tell."